High-pressure discharge lamps usually include a discharge vessel made of high-temperature resistant light-pervious material, such as quartz glass or very hard glass, forming a single discharge vessel or bulb. Two electrodes of high-temperature resistant material are melt-sealed into the discharge vessel. The discharge vessel retains a fill which includes mercury and noble gas and metal halides, as well as iodine and/or bromine, forming an excess halogen within the discharge vessel.
U.S. Pat. No. 4,138,621, Downing and Sobieski, describes a short arc discharge lamp adapted for direct current operation for use with a short arc, operated within a single uncovered discharge vessel or bulb open to the atmosphere. The bulb retains an ionizable fill including an inert gas such as argon, halogen, and a metal halide, for example indium idiode. The discharge lamp has a high average light density. The lifetime of the lamp, however, unfortunately is limited, being on the average less than 100 hours, and the color stability of the radiation emitted from the lamp is insufficient for optical projection use. The density of emitted light decreases as the lamp is used, apparently due to substantial erosion of the tip of the cathode, due to the d-c operation of the lamp.
Mercury vapor high-pressure discharge lamps of the type described, for example, in the referenced U.S. Pat. No. 3,654,506, KUHL and DOBRUSSKIN, and German Pat. No. 21 14 804, utilize an additive of rare-earth halides to the fill. Dysprosiuim and/or holmium and/or thulium are suitable rare-earth halide additives. The laps have a light emission of 90 lumens per watt (1 m/W) or more; the color temperature of the radiation is 6000.degree. K., with a color rendering index Ra of 92. These lamps are not suitable for projection use or for combination with other optical systems which require high light density, since the lamps are comparatively large, that is, they are not compact with a comparatively short arc. If the lamp is reduced in size, so that the dimensions would be suitable for projection use, the discharge arc will have a core which has a color rendition index substantially below 92 and a color temperature exceeding 6000.degree. K., this core being the only light used for illumination of a projection window, for example.